A supercharger can be implemented to supply compressed air to a combustion engine. When the air is compressed, then more air can be supplied, enabling a vehicle to produce more power. There are different kinds of superchargers available, including Comprex, Roots type, twin-screw, and centrifugal. They differ in the way that air is compressed and moved to the intake manifold of the engine.
The Roots type supercharger is a positive displacement pump that forces air around the outer circumference of rotors and blows the air into the manifold. Therefore, a Roots type supercharger is sometimes called a “blower.” More specifically, the Roots type supercharger has two counter-rotating lobed rotors. The two rotors trap air in the gaps between rotors and push it against the housing as the rotors rotate towards the outlet/discharge port into the engine's intake manifold. By moving air into the manifold at a higher rate than the engine consumes it, pressure is built.
Because of its simple design, the Roots type supercharger is widely used. However, the Roots type supercharger has some disadvantages. When the chamber of trapped air is opened to the engine's intake manifold, the pressurized air in the engine's intake manifold reverse-flows according to thermodynamic and fluid mechanic principles into the supercharger. Further, there could be a leakage of air between the rotors due to gaps, or leakage due to gaps between the rotor lobes and housing, the gaps supplied for thermal expansion tolerances. Both reversion of air and air leakage contribute to the thermal inefficiencies of the Roots type supercharge. And, due to its nature to produce high discharge temperatures, it can take away from the engine performance. For example, when the temperature of discharged air is increased, it can cause detonation, excessive wear, or heat damage to an engine.
In many positive displacement compression devices, such as reciprocating compressors, the pressure is increased by reducing the volume occupied by gas. For example, a piston physically compresses a large volume of gas into a smaller volume to increase pressure. However in a Roots device there is no mechanism like a piston to compress the gas. The Roots blower scoops the air from a low pressure suction side and moves this air to the high pressure outlet side. When the low pressure air scooped by the Roots supercharger comes in contact with the high pressure outlet side, then a backflow event takes place whereby the high pressure gas from the outlet backflows into the supercharger to compress the low pressure gas into higher pressure gas. Thus the compression of gas in the supercharger happens through this backflow event. This also heats up the compressed low pressure gas to a higher temperature based on thermodynamic principles. After compression of the gas, the blades of the Roots supercharger squeeze the compressed air out of the supercharger into the high pressure outlet side.
Typically, Roots superchargers use hot high pressure air available at the outlet for the backflow event. However, it is possible to cool the Roots compressor by using relatively colder high pressure gas available after the intercooler. But, issues remain to determine the backflow slot sizing, placement, and geometry necessary to get an optimum backflow event that provides the lowest operating temperature for the supercharger while providing the highest operating efficiency.